An electrosurgical generator is commonly used in surgical practice to perform arc cutting and coagulation. The electrosurgical generator produces a high-frequency electric current to cut tissue with limited blood loss and enhanced cutting control compared to a metal blade. Standard industry practice is for electrosurgical generators to measure and average the alternating current (AC) output power over several cycles and use a low-bandwidth control loop to adjust the duty cycle of a pulse width modulated (PWM) converter, modulating the carrier of a fixed-output-impedance resonant inverter to achieve the desired output characteristic. However, the feedback control loop and several cycles average gives rise to latency issues.
One example of an industry practice is for electrosurgical generators to mimic medium-frequency (MF) amplitude modulated (AM) broadcast transmitters via a method commonly called the Kahn Envelope Elimination and Restoration technique. Such generators typically use a class-D or class-E RF output stage operating with constant voltage amplitude at the electrosurgical analogy of a carrier frequency. In various known embodiments, the generators are combined with an efficient converter power supply amplitude modulator, sometimes referred to as a class-S modulator. The converter power supply amplitude modulator may be configured to regulate the RF output voltage, current, or power dissipated in the tissue load to a desired power versus impedance characteristic called a power curve.
The assumption of such a technique is that the tissue load changes at rates substantially lower than the audio frequency (AF) band. However, this assumption is not entirely accurate when viewed through the prism of arcing, which is the primary mechanism of cutting and coagulation in electrosurgery. Arcing in electrosurgery can extinguish and re-ignite in the middle of a cycle, and changes in its characteristics can occur on scales much broader than the AF. Therefore, this assumption may be one of convenience more so than fact, since the feedback of RF for purposes of control is well known to be very difficult due to the lag introduced by most common feedback controller techniques.
The commonly used envelope feedback regulation for electrosurgery is accomplished by measuring and averaging the alternating current (AC) output power and load impedance via voltage and current sensor feedback over many (sometimes hundreds) of cycles. This approach is complex, and its slow response during arcing leads to poor regulation of the AC output power, resulting in undesirable thermal spread or other well-known tissue damage such as charring and scarring. Thus, a need exists for an electrosurgical generator that overcomes these and other deficiencies.